In the hygiene materials, such as the paper diapers, the sanitary napkins and the incontinence pads, an absorber whose constituent materials are a hydrophilic fiber, such as pulp, and a water-absorbing resin is widely utilized to absorb body fluids.
In recent years, there exists a great need for the hygiene material which has excellent liquid diffusibility and liquid permeability, that is, which is highly functional, and is thin for improving the fit and the water-absorbing performance.
In order to improve the functions of the hygiene material, and at the same time, reduce the thickness thereof, used conventionally is a method for increasing the concentration of the water-absorbing resin in the hygiene material by (i) reducing the amount of hydrophilic fiber having low bulk specific gravity and (ii) increasing the amount of water-absorbing resin having excellent water-absorbing property and high bulk specific gravity. However, this method causes a problem in that influences on skin (skin disorders) become terrible due to, for example, residual monomers contained in the water-absorbing resin. Therefore, in order to improve the functions of the hygiene material, and at the same time, reduce the thickness thereof, it is necessary to simultaneously realize (i) improvements of the liquid diffusibility and liquid permeability of the water-absorbing resin and (ii) reduction in the amount of residual monomers.
So far, disclosed as a method for improving the liquid diffusibility and liquid permeability of the water-absorbing resin are, for example, (i) a method for realizing a product having a water-absorbing speed, a water-absorbing performance and a water-holding property in a balanced manner by impregnating a water-soluble peroxide radical initiator with a base polymer and heating the same to constitute the water-absorbing resin whose interior portion is a soft gel and whose exterior portion is a hard gel (Patent Document 1), (ii) a method for obtaining the water-absorbing resin having excellent holding ability and permeability by coating the water-absorbing resin with a polyol aqueous solution in the presence of an aqueous solution of cation (Patent Document 2), etc.
Moreover, disclosed as a method for reducing the amount of residual monomers are (i) a method for adding, to the water-absorbing resin, a reducing substance which reacts with the residual monomers in the water-absorbing resin, obtaining the water-absorbing resin having a predetermined water content, and heating the water-absorbing resin at a predetermined temperature while the changes of the water content of the water-absorbing resin is maintained in a predetermined range or lower than a predetermined range (Patent Document 3), (ii) a method for adding, in the presence of water, a sulfur atom-containing reducing substance, further adding an oxidizer, and heating the resulting mixture (Patent Document 4), (iii) a method for adding persulfate to crosslinked hydrogel and drying the resulting mixture (Patent Document 5), etc.
However, these conventional technologies cannot simultaneously realize both (i) the improvements of the liquid diffusibility and liquid permeability of the water-absorbing resin and (ii) the reduction in the amount of residual monomers. Thus, when the water-absorbing resin obtained by these conventional technologies is practically used in the hygiene materials such as diapers, physical properties and safety of this water-absorbing resin are insufficient.
That is, although the water-absorbing resin whose amount of residual monomers is small is publicly known, the water-absorbing resin which is highly safe and highly functional when used practically in the hygiene materials such as diapers has not been obtained yet. Therefore, the hygiene material which is highly safe and highly functional has not been obtained yet.
Conventionally, the water-absorbing resin whose amount of residual monomers is small is disclosed in, for example, U.S. Pat. Nos. 4,929,717, 4,90,202, 5,116,011, 5,250,640, 5,210,298, 5,338,810, 5,229,488, 5,866,678 (Patent Document 3), U.S. Pat. Nos. 6,207,796, 6,552,141 and 7,030,199, and European Patent No. 0505163. Generally, a step of reducing the amount of residual monomers becomes a cause of an increase in cost. In addition, this step of reducing the amount of residual monomers tends to deteriorate the physical properties of the water-absorbing resin. Thus, it is difficult to simultaneously obtain both the improvements of the physical properties of the water-absorbing resin and the reduction in the amount of residual monomers.
Moreover, among methods for reducing the amount of residual monomers, widely known is a method (for example, a method disclosed in Patent Document 3) for adding, as an additive, to the water-absorbing resin a water-soluble compound (for example, sulfite or bisulfite) which reacts with the residual monomers. However, since this additive is usually used in the form of an aqueous solution, there are problems in that (i) a drying treatment is required and (ii) the water content of the water-absorbing resin increases (the solid content decreases) as the amount of residual monomers decreases.
Further, in recent years, there is a demand for the water-absorbing resin having higher physical properties. That is, in addition to the improvements of a centrifuge retention capacity (CRC) and an absorbency against pressure (AAP), there is even a demand that, for example, the liquid permeability under load such as a saline flow conductivity (SFC, see U.S. Pat. No. 5,669,894, U.S. Pat. No. 6,849,665B1, U.S. Pat. No. 6,620,889B1, U.S. Pat. No. 6,605,673B1, etc.) is improved several times to several tens of times the conventional value (the liquid permeability under load is conventionally 1 (×10−7·cm3·s·g−1) to 10 (×10−7·cm3·s·g−1)).
Means for improving these physical properties (especially, SFC) is to increase a surface-crosslinking density of the water-absorbing resin. However, since long surface-crosslinking or high-temperature surface-crosslinking accelerates thermal deposition of impurities (acrylic acid oligomer (U.S. Patent No. 2006-0036043) and β-hydroxypropionic acid (U.S. Pat. Nos. 6,388,000, 6,998,447 and 7,078,458)) of raw acrylic acid to acrylic acid, the residual monomers (residual acrylic acid) increase. Thus, there is a problem in that the residual monomers (residual acrylic acid) increase by improving the physical properties (especially, SFC).
That is, it has conventionally been difficult to simultaneously obtain both high physical properties (especially, high SFC) and the reduction in the amount of residual monomers. Moreover, according to methods, disclosed in Patent Documents 3 to 5, for reducing the residual monomers, there is a problem in that the water content of the water-absorbing resin increases as the amount of residual monomers decreases.
Moreover, as the surface-crosslinking method other than the heating, known is a method for (i) mixing the water-absorbing resin with a water-soluble radical polymerization initiator and (ii) irradiating the mixture with an active energy ray (Patent Document 6). However, this method is not preferable since the amount of the water-soluble radical polymerization initiator used is very large, it is not economical, and the obtained water-absorbing resin may become brown or yellow in color.
Moreover, in this method, the amount of water used together with the water-soluble radical polymerization initiator is also very large. Further, since this method does not require heating, the water content of the obtained water-absorbing resin is kept high, but as a result, the amount of absorption per unit weight of the obtained water-absorbing resin deteriorates. Therefore, this water-absorbing resin may not be suitable for the hygienic material which contains a large amount of the water-absorbing resin. Moreover, the other properties (such as SFC) of this water-absorbing resin are also insufficient.
Moreover, as described above, although the water-absorbing resin whose amount of residual monomers is small is conventionally known, the amount of residual monomers has been evaluated by (i) dispersing the water-absorbing resin in a large excess of water (the amount of water is several hundreds of times to several thousands of times the amount of the water-absorbing resin), (ii) extracting the residual monomers, and (iii) measuring the amount of residual monomers contained in the entire water-absorbing resin. However, since the water-absorbing resin that is a form (practical use form) used practically in the hygiene materials such as diapers is not dispersed in a large excess of water but is in a dry state in which liquid urine does not exist practically, it is found that the amount of residual monomers measured by the above method using a large excess of water does not correlate with the amount of residual monomers contained in the water-absorbing resin of practical use form.
That is, it is found that (i) the amount of residual monomers measured by a conventional method does not express the amount of residual monomers exuding in the hygiene materials such as diapers and (ii) it cannot be an index of diaper rash or safety. Especially, in order to increase the amount of absorption and reduce the thickness, the amount of water-absorbing resin used in the hygiene materials such as diapers is increasing in recent years. The amount of water-absorbing resin used in the hygiene material is 50 weight % or more of an absorber layer, and in some hygiene materials, it is 70 weight % or more. Therefore, it is important to improve both the physical properties and reduce the amount of residual monomers.
Here, in addition to the conventional problems in that (i) the reduction in the amount of residual monomers leads to an increase in cost, (ii) it is difficult to simultaneously realize both the reduction in the water content and the reduction in the amount of residual monomers, (iii) it is difficult to simultaneously realize both high physical properties (especially, high SFC) and the reduction in the amount of residual monomers, etc., the present inventors has tackled a new task of providing the water-absorbing resin of practical use form whose amount of residual monomers is small.
As a result, it is found that a surface residual monomer ratio that is a new parameter truly expresses the amount of residual monomers in the water-absorbing resin of practical use form in the hygiene materials such as diapers. Moreover, the present inventors found that by (i) controlling the surface residual monomer ratio to be a value in a certain range or less than a certain range, and (ii) using the water-absorbing resin whose physical properties (especially, SFC) (the improvements of these physical properties (especially, SFC) cannot be realized simultaneously with the reduction in the amount of residual monomer) are improved to a certain level or higher, it is possible to obtain the hygiene material which can simultaneously realize both high physical properties and the reduction in the amount of residual monomers. Thus, the present inventors have completed a new particulate water-absorbing agent of the present invention.
Patent Document 1
U.S. Pat. No. 4,783,510
Patent Document 2
U.S. Pat. No. 6,605,673 B1
Patent Document 3
U.S. Pat. No. 5,866,678
Patent Document 4
Japanese Patent No. 3279401
Patent Document 5
U.S. Pat. No. 6,914,099 B2
Patent Document 6
WO2006/062258A2